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“Curvy women will live longer,” the Daily Express today reported, saying that Jennifer Lopez, Nigella Lawson and Beyonce “are in great shape to live longer” because of their “well-rounded figures”. According to the news, a study has found that a big bottom and thighs could help to prevent metabolic disorders such as heart disease, stroke and diabetes.

The reports of this study and the newspapers' interpretations are misleading, as they suggest that the study measured health outcomes of people with different body shapes. This was not the case. The study actually assessed the different ways that fat cells in the thighs and abdomen respond to over-eating. In effect, it has illustrated the known phenomenon of people putting on weight in different places in different ways.

The findings of this interesting study – that people who gained more of the weight on their thighs were less likely to have large abdominal fat cells – does not mean that women with bigger hips and a ‘pear-shape’ will live longer.

Where did the story come from?

The study was carried out by researchers from The Mayo Clinic, the Howard Hughes Medical Institute and Yale University. It was funded by a number of US institutions, including the National Institutes of Health, the US Public Health Service, the Noaber Foundation, the Mayo Foundation, and the National Institutes of Health. The study was published in the peer-reviewed medical journal Proceedings of the National Academy of Sciences (PNAS).

The news reports of this study suggest that the research compared the health of participants with different distributions of body fat. It did not. Previous studies have found that fat distribution is important and that the waist-to-hip ratio may be a better predictor of cardiovascular disease risk than BMI values: a higher ratio (more upper body fat than lower body fat) is associated with poorer health. This study actually investigated how the fat cells at these two sites respond to over-feeding.

What kind of research was this?

Studies have shown that the distribution of body fat is an important predictor of the consequences of obesity. Upper-body fat gain is associated with negative consequences, and lower-body fat gain seems to have a protective effect. The researchers say that little is known about the mechanisms that regulate the way fat accumulates.

Other studies have also suggested that from the age of 20, fat gain is likely to be the result of individual fat cells growing in size rather than increases in the numbers of fat cells. However, these findings were based only on investigations of abdominal fat cells.

In this observational study, researchers gave closer investigation to the changes in fat cells at different sites on the body seen in a group of over-feeding adults. Specifically, they were interested in whether there was a difference in the way that the upper body and lower body deposited fat.

What did the research involve?

The researchers recruited 28 healthy adults (15 male, 13 female) with an average age of 29. None of the group were obese (BMI <26kg/m2) or had a history of obesity. None of them were taking medications (except oral contraceptives).

For 10 days, three specially prepared daily meals were provided to maintain current weight. During this time, body composition was assessed using imaging (X-rays, CT scans etc). Samples of fat tissue were obtained from two regions of the body: the abdomen and the thigh.

The participants then began the over-feeding for an eight-week period during which they were instructed to eat until they were more full than usual. They were also provided with supplemental food (ice-cream shakes, chocolate bars or high-calorie drinks). Weight gain was measured regularly. After the eight-week period, participants were admitted back to the unit to have further fat biopsies and other measurements taken. They then returned to a diet that stabilised their weight.

A comparison was made of the size and number of fat cells at the upper and lower-body sites before and after over-feeding. The researchers also measured the activity of the fat cells and responses in the precursor cells that develop into fat cells.

What were the basic results?

As expected, the body weight of the group increased by an average of 4.6kg, due to an increase in fat tissue. These weight gains led to a BMI increase of 1.5kg/m2.

In general, the size of abdominal fat cells increased in response to weight gain, but their number did not. In women, those who had smaller abdominal fat cells to start with saw a greater increase in the size of the cells than those who had normal-sized cells to start with. This pattern was not seen in men.

For the thigh cells, weight gain was more linked to an increase in the number of fat cells rather than an increase in their size. The relative fat gain in the lower body was a ‘negative predictor’ of the change in the size of abdominal fat cells, i.e. people who gained more fat in the lower body were less likely to have large fat cells around their abdomen.

After further modelling, the researchers note that a gain of 1.6kg results in the creation of about 2.6 billion new fat cells within eight weeks. This response of fat cells depended on gender and the size of fat cells seen at the start of the study. For abdominal fat, weight gain seems to be due to an increase in the size of fat cells rather than in cell numbers.

How did the researchers interpret the results?

The researchers say that their results challenge the thinking that the total number of body fat cells remains constant in adults. They say that the ability of healthy adults to expand lower-body fat via hyperplasia (i.e. increasing cell numbers) may prevent or delay abdominal weight gain.

Conclusion

The newspapers have extrapolated the findings from this research to reach unsupported conclusions that “curves will make you live longer”. However, the two key findings of the research are:

Different mechanisms lie behind weight gain in the abdomen and in the thighs.

Adults who put on weight in their lower body are less likely to do so in their abdomen.

This illustrates, in an experimental situation, what is already known, i.e. that people gain weight in different places. Despite what newspaper interpretations may suggest, the study did not compare the health of those who gained weight in their thighs with those who gained more weight in their abdomen.

To note:

The study did not measure health outcomes

Weight gained (increased size of fat cells) on the abdomen appeared to occur by a different mechanism to that on the thighs

The greater the change in fat cells in the thigh, the smaller the fat cells in the abdomen. The researchers imply that this may help to explain why some people put on weight on their thighs more readily than around their abdomen.

This is interesting research that illustrates the underlying mechanisms of fat gain in two important regions of the body. It shows that the fat cells of the abdomen behave differently to those of the thigh in individuals who are over-eating. It also shows that people who gain weight in their lower body have smaller fat cells in their abdomen. Crucially, this study does not directly inform the debate about which body shape is healthier.